1. Field of the Invention
This invention relates to a magnetron used in a microwave oven to generate microwaves, and more particularly a magnetron including electromagnetic wave absorbent ferrite members provided on the outer periphery of a filament ceramics which is disposed adjacent an input section of the magnetron, so as to prevent leakage of unnecessary fundamental and higher harmonic waves (hereinafter referred to as "unnecessary electromagnetic waves").
2. Description of the Prior Art
Generally, a magnetron for a microwave oven is of the type shown in FIGS. 1 and 2 of the accompanying drawings, which includes a cylindrical anode 2 disposed within a yoke 1 serving as a frame; a cathode section comprising a coil filament 6 which is disposed about a center lead 18 to emit thermions and restrained and supported by upper and lower end shields 4,5; a power input section "A" comprising the center lead 18 and a side lead 19 and acting to apply electric power to the cathode section; an active space 7 defined by the upper and lower end shields 4,5, and a plurality of radially extending vane 3 mounted on the inner periphery of the cylindrical anode 2; a magnetic circuit section comprising magnets 8 and magnetic poles 9 and acting to apply magnetic flux into the active space 7; an output section comprising an antenna lead 10, an antenna ceramics 11 and an antenna cap 12 and acting to emit microwave energy transferred to the cylindrical anode 2 to the exterior of the magnetron; and a filter circuit section comprising a choke coil 13 and a through type condenser 14 and acting to prevent unnecessary electromagnetic waves produced in the active space 7 during oscillation of the magnetron from backflowing to the input section.
In the drawings, reference numeral 15 designates a shield case enclosing and protecting the input section and the filter circuit section, 16 designates a filament ceramics (hereinafter referred to as a "F-ceramics"), and 17 designates filament terminals (hereinafter referred to as "F-terminals").
In the magnetron thus constructed, when DC current is applied to the input section so that the magnetron performs oscillating operation, the filament 6 is heated to emit the thermions into the active space. The thermions effect cycloidal movement as they undergo the force of an electric field induced between the filament 6 and the vanes 3 and a magnetic flux applied into the active space 7 between the filament and the vanes by the magnetic poles 9 of the magnetic circuit section. The thermions thus accelerated generate microwave energy which will be received by the vanes 3. The microwave energy transferred to the vanes is discharged through the antenna lead 10 of the output section to the exterior of the magnetron so as to dielectrically heat and cook foodstuffs placed within the microwave oven. At this time, the unnecessary electromagnetic waves produced in the active space tend to be leaked out of the input section via the center lead 18 and the side lead 19.
To prevent such leakage of the unnecessary electromagnetic waves from the input section, the unnecessary electromagnetic shielding structure of a prior art magnetron comprises, as shown in FIGS. 2 and 3, the filter circuit section having the choke coil 13 and the through type condenser 14 interconnected in series, and the shield case 15 enclosing the filter circuit section, thereby shielding the unnecessary electromagnetic waves leaked out of the input section via the center and side leads.
However, the shielding structure according to the prior art fails to provide efficient and satisfactory shielding of the electromagnetic waves because the unnecessary electromagnetic wave leaked to the exterior of the magnetron via the center and side leads 18, 19 of the input section can be shielded to some extent by means of the shield case and the filter circuit section, while a substantial amount of the leaked electromagnetic waves still remain within the shield case. Further, the unnecessary electromagnetic waves may be leaked to the exterior of the magnetron through a clearance between the contact areas of the shield case 15 and the condenser 14.
As an example, when leakage of the unnecessary electromagnetic waves from the magnetron having the shielding structure as set forth above has been measured with the shield case removed, it has been found that there occurs the leakage exceeding the value of 30.about.40 dB which is the design criteria for leakage of electric waves.
The leaked unnecessary electromagnetic waves in the input section may cause a sparking phenomeonon and burn-out of the choke coil 13 in the shield case 15, and the electromagnetic waves leaked from the magnetion may cause radio disturbance.